Embodiments of the present invention relate generally to a permanent MRI magnetic field generator. More specifically, embodiments of the present invention relate to an apparatus and method for adjusting a magnetic center field of a permanent MRI magnetic field generator.
It should be appreciated that a highly uniform magnetic center field is generally preferable for magnetic resonance imaging (alternatively referred to as xe2x80x9cMRIxe2x80x9d) used as a medical device. Currently available (low maintenance) MRI systems include a permanent magnetic field generator that creates middle range uniform field (0.2 to 0.5 Tesla) in pre-determined space (alternatively referred to as xe2x80x9cimaging volumexe2x80x9d). Known permanent magnetic field generator usually use multiple small sized permanent magnet blocks (NdFeB for example) together with other iron parts to form a single magnetic object or polepiece and to achieve the desired magnetic center field with high uniformity in the imaging volume.
It should be appreciated that the accuracy of the magnetic center field is one of the critical parameters for magnets used in an MRI system or device, and thus to the imaging quality of the MRI device as a whole. Furthermore, it is known that, for a particular magnet used in an MRI system or device, the magnetic center field must be stable, normally staying within a small range (i.e., having a very small tolerance) to produce good quality images, where such variance may be due to such factors as the adjustability of radio frequency (alternatively referred to as xe2x80x9cRFxe2x80x9d) coils. For example, in a 0.35 Tesla MRI system, the magnetic center field should normally stay within the range of 0.3495 to 0.3510 Tesla in order for the system to produce good quality medical images.
It should be appreciated that assembling one or more magnetic members together to form a magnet used in an MRI system having a targeted magnetic center field is very difficult. That is due to the magnetic center field varies due to a number of factors, including, without limitation, material physical tolerance, variation of material physical properties, magnetic interaction forces, assembly tolerances, process variations, etc. It is known that after assembly of the MRI system, the magnetic center field of the MRI deviates off the mean. Frequently, the magnetic fields of the MRI system must be adjusted in order to achieve the desired magnetic center field.
Providing an MRI system that enables adjusting the permanent magnets after assembly, enables more permanent magnetic blocks to be installed in MRI systems, as enabling adjusting the permanent magnets after installation enables the magnetic center field to be raised or otherwise adjusted if the magnetic center field is determined to be below a desired design value.
It is known to add one or more permanent blocks in order to obtain a desired magnetic center field. However, it is much more difficult (and sometimes is impossible) to extract installed magnetized blocks as the magnetized blocks are often either glued to the neighboring parts or clamped to the yoke after insertion to fix the blocks thereto due to large repelling magnetic force between magnetic parts.
Furthermore, installing more permanent blocks, enables adjusting the magnetic center field discretely as desired, and not just as fraction of the permanent magnetic block. For example, if system requires a 70 Gauss increase, however, each permanent magnetic block increases the magnetic field by 45 Gauss. In this situation, adding one block set or two sets will not achieve desired center magnetic field.
Embodiments of the present invention relate to a magnetic field generator for an MRI. More specifically, embodiments of the present invention relate to a method for adjusting of center magnetic field, magnetic field generator for MRI incorporating permanent magnets. At least one embodiment of this invention relates to an approach to adjust magnetic field in the imaging volume used for MRI magnetic field generator using a mechanical design.
One embodiment of the presenting invention relates to a magnetic field generator system having a magnetic center field. This embodiment comprises opposing posts and opposing yokes connected to the opposing posts, where at least one of the posts has a spacer. At least one permanent magnetic block is connected to at least one of the opposing yokes. Further, at least one spacer is formed in at least one of the posts, the yokes and/or the permanent magnetic block.
In one or more embodiments of the present invention, the at least one spacer comprises at least one slot formed in at least one of the opposing yokes. The at least one spacer may further comprise at least one slot formed in opposing ends of at least one of the opposing slots and at least one slot formed in each of the opposing yokes. It is further contemplated that the spacer may comprise a plurality of slots formed in at least one of the opposing yokes in a symmetrical or un-symmetrical pattern. It is further contemplated that at least two of the plurality of slots are of different sizes and/or shapes.
In at least one embodiment, the at least one spacer comprises at least one plate, a steel plate for example, adapted to be inserted at least part way in at least one of the one slots. It is contemplated that the spacer comprises a plurality of plates, wherein at least two of the plates are comprised of differential. It is also contemplated that the at least one plate is composed of the same or different material than the opposing yokes.
One embodiment of the present invention comprises a method for adjusting a magnetic center field of a permanent magnet system. In at least this embodiment, the method comprises determining if an adjustment required for the magnetic center field and adjusting the magnetic center field by using at least one spacer in the permanent magnet system.
One or more embodiments of the method comprises adjusting the magnetic center field using the at least one spacer comprises forming at least one slot in at least one of two opposing yokes. Adjusting the magnetic center field may further comprise forming at least one slot in opposing ends of at least one of the opposing slots and at forming at least one slot formed in each of the opposing yokes. It is further contemplated that adjusting the center field may comprise forming a plurality of slots formed in at least one of the opposing yokes in a symmetrical or un-symmetrical pattern. It is further contemplated that adjusting the center field comprises forming a plurality of slots, wherein at least two slots of the plurality of slots are of different sizes and/or shapes.
In at least one embodiment, adjusting the magnetic center field using at least one spacer comprises inserting at least one plate, a steel plate for example, at least partway into at least one of the slots. In one embodiment, the steel plate is completely inserted into the slot. It is further contemplated that adjusting the magnetic field using at least one spacer comprises inserting a plurality of plates, at least partway in one or more of the slots, wherein at least two of the plates are comprised of the same or different material. It is also contemplated that the at least one plate is composed of the same or different material than the opposing yokes. If a plurality of plates are use, one or more plates may-be inserted partway into a slot, while one or more plates may be completely inserted into the slots. It is further contemplated that adjusting the center field comprises a plurality of plates, where at least two plates of the plurality of plates are different sizes and/or shapes.
Still another embodiment of the present invention comprises a method for adjusting a magnetic center field of a permanent magnet system in an MRI device. In this embodiment, the method comprises determining an adjustment required for the magnetic center field and adjusting the magnetic center field using at least one spacer in the permanent magnet system. In this embodiment, the method further comprises determining if the adjustment is sufficient.
One or more embodiments of the method comprises adjusting the magnetic center field using the at least one spacer comprises forming at least one slot in at least one of two opposing yokes. Adjusting the magnetic center field may further comprise forming at least one slot in opposing ends of at least one of the opposing slots and at forming at least one slot formed in each of the opposing yokes. It is further contemplated that adjusting the center field may comprise forming a plurality of slots formed in at least one of the opposing yokes in a symmetrical or un-symmetrical pattern. It is further contemplated that adjusting the center field comprises forming at least two of the plurality of slots are of different sizes and/or shapes.
In at least one embodiment, adjusting the magnetic center field using at least one spacer comprises inserting at least one plate, a steel plate for example, at least partway into at least one of the slots. It is further contemplated that adjusting the magnetic field using at least one spacer comprises inserting a plurality of plates, at least partway in one or more of the slots, wherein at least two of the plates are comprised of the same or different material. It is also contemplated that the at least one plate is composed of the same or different material than the opposing yokes.